1. Field of the Invention
The present invention relates generally to the field of electronics, and specifically to the aspects of power supply management in electronic devices, more particularly in relation to the use of the USB (Universal Serial Bus) interface for the interconnection of mobile, battery-operated devices with other devices.
2. Discussion of the Related Art
In the last few years, mobile, battery-operated digital electronic devices like mobile (cellular) phones, digital cameras, camcorders, PDAs (Personal Digital Assistants) have experienced a massive diffusion through users.
The success gained by the USB as a means for the connection of Personal Computers (PCs) to PC peripherals like printers, keyboards, pointing devices, memory card readers, pen drives, just to cite some, has driven producers of consumer electronics devices to equip mobile devices with USB interfaces.
However, the USB standard (like, for example, the USB 2.0 specification) originally defined a host/peripheral relationship scheme wherein the PC is always the host, and devices that are plugged thereinto are always peripherals. In this interconnection scheme, the PC supplies the USB voltage to the peripherals connected thereto.
The desire of making it possible to directly connect one to another mobile electronic devices, without the need of a PC acting as a host (a use not supported by the original USB standard), has brought to the definition of a supplement specification to the USB standard, the so-called “USB OTG” (On-The-Go), which defines how two generic electronic devices, like for example two mobile electronic devices, connect to each other without the necessity of a PC.
Without entering into excessive details, well known to those skilled in the art, according to the USB OTG prescriptions, two device configurations are defined: “A-devices” and “B-devices”. A-devices are devices that, by default, act as hosts when connected to other devices; B-devices are instead devices that act by default as peripherals. A USB OTG device can be either an A-device or a B-device, thus it has the capability of acting as a host or as a peripheral, based on a negotiation between the USB OTG device and the device(s) to which it is connected through the USB interface.
When a USB OTG device acts as a host, it is required to provide the USB voltage supply to the peripheral USB devices connected to it. The USB voltage supply is a voltage in the range from approximately 4.85 V to approximately 5.25 V. Since battery-operated mobile electronic devices are typically powered by a lithium (lithium ion or lithium polymer) battery, which provides a voltage in the range from approximately 3.0 V to approximately 4.2 V, a USB OTG device has to include a voltage step-up circuit arrangement, to increase the battery voltage to the prescribed USB voltage rating.
Voltage step-up circuits are commonly of the switched-mode type, and include one or more inductors.
Mobile electronic devices are significant users of battery power; for example, the several different and more and more complex multimedia functions that are currently implemented by a mobile phone have a significant impact on the device's power consumption. Thus, the device battery should be re-charged as quickly and as often as possible. Charging the mobile device battery through the USB interface when the device is connected to other devices acting as hosts, like for example a PC, and thus supplying the USB voltage supply, significantly contributes to maintaining charged the battery of the mobile device. To this purpose, a buck charger including a DC-DC voltage down-converter needs to be embedded in the mobile device.
Also DC-DC voltage down-converter circuits are commonly of the switched-mode type. Thus, USB OTG mobile electronic devices should include two or more inductors (and the respective switched-mode drivers), one for the voltage step-up and the other for the voltage down-conversion.
Inductors are relatively expensive, and are normally in the form of coils, i.e. discrete components, thus they occupy a non-negligible area in a printed circuit board.
This is in contrast to the trend toward reduction in size and costs of the mobile electronic devices.
The Applicant has observed that, in a battery-powered, mobile USB OTG device, the two functions, namely the DC-DC voltage down-conversion for charging the mobile device battery via the USB, exploiting the USB voltage supply supplied by a USB host connected to the USB OTG device, and the voltage step-up function, for increasing the battery voltage to the prescribed USB voltage value, to be supplied to a USB peripheral connected to the USB OTG device, are essentially never active at the same time. Indeed, when the mobile device acts as a USB host, it is responsible for supplying the USB voltage to the other USB device(s) connected thereto as peripherals, and cannot use the USB voltage to re-charge the battery; on the contrary, when the mobile device is not the host, it can exploit the USB voltage supplied by the host USB device to which it is connected to re-charge the battery.